For example, in a wireless communications system, wireless signals are transmitted from a transmitter in a communication device for transmission and then received by a receiver of a communication device for reception.
FIG. 6 shows an exemplary configuration of a conventional transmitter. Herein, like parts as shown in FIG. 1, which will be described later in conjunction with an embodiment of the present invention, are designated by like reference numerals without limiting the scope of the present invention.
The transmitter shown in FIG. 6 has a limiter function for transmission signals and is capable of perform distortion compensation function for an amplifier 7.
As the distortion compensation for the amplifier 7, various methods to compensate distortion caused by the nonlinear characteristics of amplifiers may be used.
A limiter 41 and a LPF (Low Pass Filter) 42 different from those in FIG. 1 will now be described to explain a conventional method of peak suppression.
A modulated signal inputted from a modulator 1 to the limiter 41 via a multiplier 2 is delayed by a delay unit 51, and then is fed to both a peak suppression filter 53 and a peak detection unit 52. A peak of the modulated signal is detected by the peak detection unit 52, and the detection result thereof is sent to the peak suppression filter 53. Then, the modulated signal whose peak is suppressed by the peak suppression filter 53 based on the detection result is outputted to the LPF 42. Thereafter, an output signal from the LPF 42 is fed to a D/A (Digital to Analog) converter 4 and a delay unit 17.
Here, for example, according to a digital modulation/demodulation scheme with a non-constant envelope such as a QPSK (Quadrature Phase Shift Keying) scheme or a CDMA (Code Division Multiple Access) scheme, a peak may occur as shown in FIG. 7, which results in an amplifier distortion of the amplifier.
FIG. 7 schematically shows an exemplary carrier signal with a peak power. The horizontal axis indicates the time and the vertical axis indicates the amplitude (instantaneous power).
Further, FIG. 7 also shows the peak power and the average power.
In order to reduce such a peak as shown therein, a limiter for suppressing a peak component in the baseband or in the IF (Intermediate Frequency) band has been proposed, and has been used in the digital modulation/demodulation method with a non-constant envelope.
Meanwhile, frequency utilization efficiency is recently considered more important than amplifier efficiency. For example, it is not proper to use the conventional limiter for, e.g., a wideband CDMA scheme or to an OFDM (Orthogonal Frequency Division Multiplexing) scheme of high frequency efficiency, because the limiter may generate distortion to cause frequency interference, which significantly deteriorates wireless characteristics in the OFDM scheme. In such a case, a signal having an unsuppressed peak is inputted to the amplifier. To prevent this, a sufficiently large backoff is needed, which lowers the amplifier efficiency noticeably.
As a specific example, a mobile phone according to a PDC (Personal Digital Cellular) method as the second generation technology uses π/4 QPSK modulation and has the PAPR (Peak Average Power Ratio) of about 3 dB, whereas an OFDM modulated wave used in a wireless LAN (Local Area Network) or in a digital TV has a peak of 10 dB or more. Further, since the third (and later) generation mobile phone technology uses a wider band, it is expected that the PAPR will further increase.
(Patent Document 1) Japanese Patent Laid-open Application No. 2002-044054
As described above, there are demands for a transmitter having a limiter capable of suppressing a peak without generating frequency interference even for a high PAPR.
In response thereto, it has been suggested that the conventional limiter be improved to cope with a high PAPR. For example, in the transmitter shown in FIG. 6 suppresses a peak, and then the LPF 42 eliminates distortion. However, in this configuration since a peak may be further generated after a transmission signal passes through the LPF 42. To solve this problem, it has been proposed to provide a plurality of limiter 41 and a plurality of LPF 42 in a repetitive manner for performing the operations repeatedly.
However, the processing in this manner is too complicated, and it is difficult to install a transmitter with such a configuration.